The present invention relates to a process. In particular, the present invention relates to a process for synthesising nanocrystalline materials, such as nanocrystalline CdSe.
Nanocrystalline materials, which are sometimes referred to as nanoparticles, Q-particles, quantum dots or nanocrystallites, have been recognised as suitable systems for studying the transition from the molecular to the macrocrystalline level and have been extensively studied in the recent years. (D. Duonghong et al., J. Am. Chem. Soc. 1982, 104, 2977, R. Rossetti et al., J. Chem. Phys., 1984, 80, 4464, A. Henglein, Chem. Rev., 1989, 89, 1861, M. L. Steigerwald and L. E. Brus, Acc. Chem. Res., 1990, 23, 183, Y. Wang and N. Herron, J. Phys. Chem. 1991, 95, 49, H. Weller, Adv. Mater. 1993, 5, 88, A. Hagfeldt and M. Gratzell, Chem. Rev. 1995, 95, 49, L. E. Brus, J. Chem. Phys. 1984, 80, 4403, L. Brus, J. Phys. Chem. 1986, 90, 2555, P. E. Lippens and M. Lannoo, Phys. Rev. B 1989, 39, 10935 and Y. Nosaka, J. Phys. Chem. 1991, 95, 5054.)
Interest in research into new synthetic routes for semiconductor nanocrystallites is now enhanced as devices based on such materials have been fabricated. (V. L. Colvin, et al., Nature 1994, 39, 10935, B. O. Dabbousi et al., Appl. Phys. Lett., 1995, 66, 1317 and R. S. Urquhart et al., Langmuir, 1995, 11, 1127.) A number of synthetic methods have been reported for the preparation of a wide range of semiconductor nanoparticles (D. Duonghong et al., J. Am. Chem. Soc. 1982, 104, 2977, R. Rossetti et al., J. Chem. Phys., 1984, 80, 4464, A. Henglein, Chem. Rev., 1989, 89, 1861, M. L. Steigerwald and L. E. Brus, Acc. Chem. Res., 1990, 23, 183, Y. Wang and N. Herron, J. Phys. Chem. 1991, 95, 49, H. Weller, Adv. Mater. 1993, 5, 88, A. Hagfeldt and M. Gratzell, Chem. Rev. 1995, 95, 49, Y. Wang and N. Herron, J. Phys. Chem. 1987, 91, 257, H. J. Watzke and J. N. Fendler, J. Phys. Chem., 1987, 91, 854, P. C. Sercel et al., Appl. Phys. Lett. 1992, 61, 696, V. Sankaran et al., Chem. Mater. 1993, 5, 1133, A. Mews et al., J. Phys. Chem. 1994, 98, 934, O. V. Salata et al., Appl. Phys. Lett. 1994, 65, 189, M. L. Steigerwald et al., J. Am. Chem. Soc. 1998, 110, 3046, A. R. Kortan et al., J. Am. Chem. Soc. 1990, 112, 1327 and J. G. Brerman et al., J. Am. Chem. Soc. 1989, 111, 4141.)
Known processes for preparing nanocrystalline materials, such as nanocrystalline CdSe, have included arrested precipitation in micelles (M. L. Steigerwald et al) or the reaction of molecular species at high temperature in organic solvents. (A. R. Kortan et al., J. Am. Chem. Soc. 1990, 112, 1327, J. G. Brenman et al., J. Am. Chem. Soc. 1989, 111, 4141, C. B. Murray et al. J. Am. Chem. Soc., 1993, 115, 8706 and J. E. Bowen Katari et al., J. Phys. Chem. 1994, 98, 4109.)
In more detail, Murray et al report on the preparation of CdE (where E is S, Se or Te) by the pyrolysis of two organometallic reagents by injection into a hot coordinating solvent. In particular, the Murray process involves injecting a solution of (CH3)2Cd in TOP (tri-n-octylphosphine) into a hot solution of TOP containing Se (TOPSe and TOP). Alternatively, any one of (TMS)2S (bis(trimethylsilyl)sulphide), (TMS)2Se (bis(trimethylsilyl)selenide), and (BDMS)2Te (bis(tert-butyldimethylsilyl)tellurium) may be used instead of TOPSe.
In the Murray process (CH3)2Cd is chosen as the only Cd source. Moreover, Murray et al state that (TMS)2Se or TOPSe and TOPTe are selected as chalcogen sources with TOPSe and TOPTe preferred due to their ease of preparation and their stability.
Chemical reactions in TOPO (tri-n-octylphosphine oxide) are also described by (C. B. Murray et al.). These processes have been used to prepare nanocrystallites of II/VI semiconductors (V. L. Colvin et al, B. O. Dabbousi et al, C. B. Murray et al and J. E. Bowen Katari et al.). In this instance, TOPO is used as dispersing medium and a metal source (e.g Cd(CH3)2) and a chalcogenide source (e.g. TOPSe) are injected into the hot TOPO (typically at 250xc2x0 C.) to form CdSe nanocrystallites. The size distribution of the semiconductor can be controlled by the temperature of heating during the synthesis and by size selective precipitation of the final material. (C. B. Murray et al and J. E. Bowen Katari et al.)
A refinement of the Murray process has been proposed by (J. E. Bowen Katari et al). As with the Murray process, in the Katari process CdE is prepared by the pyrolysis of two organometallic reagents by injection into a hot coordinating solvent. In the Katari process Se is dissolved in TBP (tributylphosphine) to which (CH3)2Cd is then added. The resultant (CH3)2Cd/Se solution is then added to a heated solution of TOPO.
As with the Murray process, in the Katari process (CH3)2Cd is chosen as the only Cd source.
There are however problems associated with the prior art processes for preparing nanocrystalline materials. For example, both the Murray process (C. B. Murray et al) and the Katari process (J. E. Bowen Katari et al) involve the use of hazardous chemicals, in particular (CH3)2Cd. In this regard, (CH3)2Cd is toxic, volatile and extremely difficult to handle. Moreover, on exposure to air it undergoes spontaneous combustion.
Aside from using the hazardous compound Cd(CH3)2 (V. L. Colvin et al and B. O. Dabbousi et al) to prepare nanocrystalline CdSe, other workers have used the equally hazardous H2Set (R. S. Urquhart et al) for the synthesis of the CdSe.
The present invention seeks to overcome the problems associated with the prior art processes for making nanocrystalline materials.
According to a first aspect of the present invention there is provided a process for preparing a nanocrystalline material comprising at least a first ion and at least a second ion different from the first ion, and wherein at least the first ion is a metal ion, the process comprising contacting a metal complex comprising the first ion and the second ion with a dispersing medium suitable to form the nanocrystalline material and wherein the dispersing medium is at a temperature which allows formation of the nanocrystalline material by pyrolysis when contacted with the metal complex.
According to a second aspect of the present invention there is provided a nanocrystalline material obtained by the process of the present invention.
According to a third aspect of the present invention there is provided a device comprising a nanocrystalline material obtained by the process of the present invention.
Preferably the metal ion is a divalent metal ion or a trivalent metal ion.
Preferably the metal ion is selected from a cadmium ion, a zinc ion, a lead ion, a mercury ion, an indium ion and a gallium ion, including combinations thereof.
Preferably the second ion is selected from an oxide ion, a selenide ion, a sulphide group, a phosphide group or an arsenide ion, or combinations thereof.
Preferably the second ion is or is part of a thiol-carbamate group or a seleno-carbamate group.
Preferably the second ion is or is part of a dithiol-carbamate group or a diseleno-carbamate group.
Preferably the metal complex additionally comprises an organic group and/or thio group. The organic group can be an alkyl group or an aryl group, which may be substituted.
Preferably the organic group is an alkyl group, which may be substituted and/or unsaturated.
Preferably the organic group is a dialkyl group, which may be substituted and/or unsaturated, and/or wherein the thio group is a dithio group.
Preferably the organic group is a di-C1-6alkyl group and/or the thio group is a dithio group or a diseleno group.
Preferably the organic group is a diethyl group.
Preferably the dispersing medium is at a temperature of 250xc2x0 C. or more, preferably about from 300xc2x0 C. to 350xc2x0 C.
Preferably the dispersing medium passivates the surface of the nanocrystalline material.
Preferably the dispersing medium is TOPO, or a related coordinating medium, including combinations thereof. Another dispersing medium could be TBP.
Preferably the nanocrystalline material comprises or is selected from any one of cadmium selenide, cadmium sulphide, zinc selenide, zinc sulphide, indium phosphide and gallium arsenide, including ternary and quaternary combinations thereof.
Preferably the nanocrystalline material is cadmium selenide.
Preferably the metal complex is diethyl diselenocarbamato cadmium or dithio diselenocarbamato cadmium, or related mixed alkyl complexes thereof.
Preferably the device is an optical device.
Preferably the device is any one of a non-linear optic device, a solar cell or an LED.
Preferably the device is an LED.
Preferably the device is a blue LED.
The present invention is therefore based on the surprising finding that nanocrystalline materials can be prepared by using as a reactant a metal complex which provides at least two of the ions of the nanocrystalline material. The process of the present invention is therefore very different to the Murray process (C. B. Murray et al) and the Katari process (J. E. Bowen Katari et al) wherein in each of those processes it is necessary to use two independent sources to provide at least two of the ions of the nanocrystalline material. Thus, the use of a molecular precursor containing both elements in the present process provides an attractive route to metal selenides, especially if a large scale preparation is anticipated.
The present invention is further advantageous over the prior art processes as it does not rely on the use of hazardous chemicals such as (CH3)2Cd.
The present invention is further advantageous as it provides a low cost route to prepare photovoltaic materials and optoelectronic materials, preferable examples of which include non-linear optic devices, solar cells and LEDs.
Thus the present invention shows that a single source can be used in a dispersing medium, such as TOPO, to replace the use of the hazardous metal alkyls. In a highly preferred embodiment, the present invention provides the synthesis of CdSe nanocrystallites using methyl diethyldiselenocarbamato cadmium (II) MeCddsc: [(CH3)CdSe2CN(C2H5)2]2) as a precursor. The synthetic method of this preferred embodiment is diagrammatically illustrated in FIG. 1, which makes no efforts to represent a mechanistic pathway.
Even though the pathway shown in FIG. 1 is for the synthesis of CdSe it is to be understood that the process of the present invention is useful for preparing a series of nanocrystalline materials.
Examples of nanocrystalline materials that can be prepared using an appropriate single molecule precursor can be represented by the general formulae A and B as shown below.
MIIExe2x80x83xe2x80x83GENERAL FORMULA A
wherein M is Zn, Cd, Hg or a divalent transition metal; and wherein E is O, S, Se, P, or As.
MIIIxEyxe2x80x83xe2x80x83GENERAL FORMULA B
wherein M is Al, In, Ga or a trivalent transition metal; and wherein E is O, S, Se, P, or As; and wherein x and y are appropriate intergers.
Formulae A and B also encompass related ternary systems.
Therefore, examples of nanocrystalline materials other than cadmium selenide include cadmium sulphide, zinc selenide, zinc sulphide, indium phosphide and gallium arsenide.
The general formula of the metal complex for use in the process of the present invention can be represented as:
MLnxe2x80x83xe2x80x83FORMULA I
wherein M represents a metal ion; L represents one or more ligands which need not be the same; n represents the valency of the metal; and wherein M is the first ion of the nanocrystalline material and at least one L provides the second ion for the nanocrystalline material.
Typically M is a divalent metal ion or a trivalent metal ion, such as any one of cadmium, zinc, lead, mercury, indium gallium, including combinations thereof.
Typically L is any one of an oxide ion, a selenide ion, a sulphide group, a phosphide group or an arsenide ion, or combinations thereof. More in particular L is or is part of any one of a thiol-carbamate group or a seleno-carbamate group, such as a dithiolcarbamate group or a diseleno-carbamate group.
In a preferred embodiment, at least one L is an organic group and/or a thio group. If at least one L is an organic group then preferably that organic is an alkyl group, which may be substituted and/or unsaturated, such as a C1-10 (preferably C1-6, more preferably C1-4) alkyl group, which may be substituted and/or unsaturated.
Preferably, at least one L is a dialkyl group, which may be substituted and/or unsaturated, and/or wherein the thio group is a dithio group. Preferably, the organic group is a di-C1-6 alkyl group and/or the thio group is a dithio group or a diseleno group. In a highly preferred embodiment, at least one L is a diethyl group.
Typical general formulae for suitable metal complexes containing at least one organic group for use as single molecule precursors in the process of the present invention are shown below as Formula II (for metals that are divalent) and as Formula III (for metals that are trivalent):
[RIIxe2x88x92MIIxe2x88x92(ExCNRRI)y]zxe2x80x83xe2x80x83FORMULA II
[(RII)(RIII)xe2x88x92MIIIxe2x88x92(ExCNRRI)y]zxe2x80x83xe2x80x83FORMULA III
wherein R, RI, and RIII independently represent an aryl or alkyl group as defined above, which may be substituted and/or unsaturated; MII is a divalent metal ion; MIII is a trivalent metal ion; E is any one of an oxide ion, a selenide ion, a sulphide group, a phosphide group or an arsenide ion, or combinations thereof (such as, by way of example, xe2x80x94Oxe2x80x94Sxe2x80x94); x is an integer, preferably 2; y is an integer; and z is an integer, usually 1 or 2.
As mentioned above, a highly preferred metal complex containing at least one organic group for use as a single molecule precursor in the process of the present invention is methyl diethyldiselenocarbamato cadmium (II) (MeCddsc) wherein R is C2H5; RI is C2H5; RII is (CH3); M is CdII; E is Se; x is 2; y is 1; and z is 2.
However, other preferred metal complexes containing at least one organic group for use as single molecule precursors in the process of the present invention include
Mxe2x88x92(E2CNAlk2)nxe2x80x83xe2x80x83FORMULA IV
wherein n is 2 for metals such as zinc, cadmium and lead; n is 3 for metals such as gallium or indium; E is S or Se; and A is an aryl or alkyl group, preferably ethyl; including carbamate (i.e. O-donors) thereof:
and either
RIIxe2x88x92Mxe2x88x92(E2CNA2)nxe2x80x83xe2x80x83FORMULA V
or
(RII)nxe2x88x92Mxe2x88x92(E2CNA2)xe2x80x83xe2x80x83FORMULA VI
wherein n is 1 for metals such as zinc, cadmium and lead; n is 2 for metals such as gallium or indium; E is S or Se; A is an aryl or an alkyl group, preferably ethyl; and RII is independently selected from an alkyl or aryl group as defined above, such as methyl.
Other possible metal complexes for use as single molecule precursors in the process of the present invention include related thiolates, thiophosphinates or phosphinochalcogens and related selenium containing compounds.